The present disclosure relates to an image forming apparatus including a photosensitive drum, a photosensitive drum unit provided in an image forming apparatus, and a bush member used for supporting a photosensitive drum.
An image forming apparatus of an electrophotographic type includes a photosensitive drum on which a toner image is formed by a developing device. The toner image formed on the photosensitive drum is transferred to a sheet member by a transfer device. The photosensitive drum is rotatably supported by a frame or the like. In one example of known structures, drive receiving portions are provided at both ends of the photosensitive drum, and shaft portions of the drive receiving portions are rotatably supported. In another example of known structures, in order to reduce resistance with a bearing portion, the shaft portion is rotatably supported via a ring-shaped bush member formed of a highly slidable resin material.
In general, in the case of rotatably supporting the shaft by the bearing portion, so-called runout occurs in which the center of the shaft is shifted in the radial direction relative to the center of the bearing portion. In the case where the bush member is used for supporting the shaft portion, the bush member expands or contracts in the radial direction due to variation in the ambient temperature. Therefore, the geometric tolerance (runout tolerance) for the shift amount of the runout is greater than that in the case of not using the bush member. Conventionally, under the assumption that the outer diameter size of the bush member increases due to expansion, the dimensions of the gap (space where the bush member is stored) between a shaft portion of a supported subject and the bearing portion are designed.